In a conventional magnetron, as shown in FIG. 1, a pair of permanent magnets 1a, 1b are disposed outside an anode cylinder (i.e. shell) 2 of antimagnetic and highly electrically conductive material, and they supply a magnetic field to an operative space 3 surrounded by anode vanes 4 through a pair of yokes 5a, 5b and circular magnetic pieces 6a, 6b. The vanes 4 are directly attached to the anode cylinder 2, and a pair of yokes 5a, 5b support the permanent magnets 1a, 1b. Circular magnetic pieces 6a, 6b support the anode cylinder 2. There is a directly heated spiral cathode 7 at the center of the operative space 3. As the size of the magnetron increases the magnetic resistance also increases because the magnetic circuit becomes long. Therefore, the permanent magnets 1a, 1b must be made large in order to supply the necessary desired magnetic field to the operative space 3. Furthermore, the area of the external wall of the anode cylinder 2 to which the usual heat radiative cooling fins may be attached is restricted by the existence of the permanent magnets 1a, 1b.
An improved magnetron for overcoming the defects described above, is the so-called internal magnet magnetron which is made by constructing the anode cylinder of magnetic material, locating a pair of permanent magnets in the anode cylinder, which is also a vacuum container, and making the anode cylinder a part of the magnetic circuit. This magnetron has advantages in that it is possible to reduce the size of the permanent magnets because the gap between the magnets and the operative space may be made small and the magnetic field may be intensified. Further, the leakage of the magnetic flux is made almost zero because the anode cylinder is a vacuum container and a part of the magnetic circuit itself. However, an internal magnet magnetron has a disadvantage in that the losses of the cavity resonator, which comprises anode vanes and the anode cylinder, at high frequency becomes large because the anode cylinder is made of magnetic material like iron.